The goal of the proposed research is to carry out a detailed examination of the course of vocabulary acquisition when bimodal input is made available to very young retarded children. To this end, twelve children selected from an all-day nonresidential stimulation program will be provided with bimodal (simultaneous communication) language intervention. All of the children will be in the moderate range of retardation. Six of them will be at risk for delayed speech due to Down's syndrome. The other six children will be matched on relevant variables, except that their speech will be DA-appropriate at the onset of their participation. During daily half-hour sessions of individual intervention, each child will be exposed to the same target vocabulary of word/sign pairs (e.g., the sign CUP and the word "cup" in simultaneous presentation). Special vocabularies of invented words and signs for unfamiliar referents will be included, with the purpose of disentangling the effects of previous experience with the labels and referents from the effects of the current training. A continuous record of each child's responses will be made using a highly compact computer, supplemented by periodic assessments and videotaped sessions. Data analysis will focus on tracing and comparing the children's developing vocabularies in the two modalities, with special attention to three claims: (1) that all children will show a similar relation between DA and sign acquisition, but not between DA and word acquisition; specifically, the children with Down's syndrome will show a sign advantage due to delayed speech; (2) that for speech-delayed children, their sign vocabularies will serve as a base for the later development of work vocabularies; and (3) that for speech-delayed children, their sign vocabularies will function similarly to early word vocabularies for other children. The findings will have implications for the general theory of signification as well as for the design of language intervention programs.